Editing Dalton (unit) (section)

=== Origin of the concept ===

[[File:Jean Perrin 1926.jpg|right|thumb|Jean Perrin in 1926]]
The interpretation of the [[law of definite proportions]] in terms of the [[atomic theory of matter]] implied that the masses of atoms of various elements had definite ratios that depended on the elements.  While the actual masses were unknown, the relative masses could be deduced from that law.  In 1803 [[John Dalton]] proposed to use the (still unknown) atomic mass of the lightest atom, hydrogen, as the natural unit of atomic mass.  This was the basis of the [[standard atomic weight|atomic weight scale]].<ref name=petley1989/>

For technical reasons, in 1898, chemist [[Wilhelm Ostwald]] and others proposed to redefine the unit of atomic mass as {{sfrac|1|16}} the mass of an oxygen atom.<ref name=hold2004/> That proposal was formally adopted by the [[Commission on Isotopic Abundances and Atomic Weights|International Committee on Atomic Weights]] (ICAW) in 1903. That was approximately the mass of one hydrogen atom, but oxygen was more amenable to experimental determination. This suggestion was made before the discovery of isotopes in 1912.<ref name=petley1989/> Physicist [[Jean Baptiste Perrin|Jean Perrin]] had adopted the same definition in 1909 during his experiments to determine the atomic masses and the [[Avogadro constant]].<ref name=perrin1909/> This definition remained unchanged until 1961.<ref name=chang2005/><ref name=kelt2008/> Perrin also defined the "mole" as an amount of a compound that contained as many molecules as 32 grams of oxygen ({{chem|O|2}}). He called that number the [[Avogadro number]] in honor of physicist [[Amedeo Avogadro]].